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n LJ 75 The design depths for the channels are a summation of normal depth, wave height, and required freeboard. The resulting design depths are presented in Table 21. Design depths based on both the 10- and 25-year storms are provided. In most cases, the difference in design depths for the 10- and 25-year storms are small. Where it is possible, the 25-year depths should be adopted. This allows a much larger margin of safety. The riprap was designed for the 10-year event, and may experience some damage during the 25-year and 100-year events. The riprap requirements for the same factor of safety for a 25- or 100-year event mould be impractical. However, by not letting the flow overtop the riprap I for the 25-year event, damage will be minimized during the larger event. This is why choosing the 25-year depths in the lined channels is reasonable. The C overbank slopes on all restored channels should b< 10:1. If this slope is ex- tended far enough, the 100-year peak discharge will be contained within the t" floodplain. As shown in Figure 9, riprap protection should extend a minimum ~ of five feet onto the overbanks. Fabric filter is recommended only for channel units PH-CH-15 and PM-CH-17. All restored channels within the Magpie drainage I should have a minimum of 12 inches of gravel Filter below the riprap. The proposed extent of mining shows disturbance of a very short portion of the subvatershed SW-12 drainage channel (approximately 100 feet). The pro- posed mining boundary goes just up to the channel. Here, through use of careful mining practices, the disturbance to the channel can be held to a minimum and restoration of the side slope will only be required. However, if the channel is disturbed, the length of disturbance is very short and this ' portion of the channel can be restored by matching upstream and downstream natural channel characteristics. The disturbed or restored portion of the channel should be protected by comparable riprap. 6.4.4 Comments on Channel Design and Restoraticn As can be seen from the proposed designs of major diversion channels PM-CH-15 and PM-CH-17 and the restoration channels, extensive gradation and riprap will be required. The restoration channels have been designed based on natural channel shapes and gradients. The need for riprap protection on the restoration channels vas briefly discussed. The reason is that in the natural condition (premining) most of the channels had significant vegetation . in the form of scrub oak and cedar and mature evergreen trees. Also, the chan- nels were partially armored and had large boulders throughout the length of